This invention relates to wavelength-division multiplexing in optical systems, and more particularly to apparatus for combining and/or separating combs of equally spaced wavelength channels.
An important function that must be provided in high quality optical networks is that of wavelength multiplexing and demultiplexing a plurality of signals of different wavelengths. In particular, an important device for performing this function is the interleaver (also referred to herein as a slicer) which separates an input signal consisting of closely spaced wavelength signals, into two sets of equally spaced wavelength channels, each set having twice the spacing of the interleaved signal. The interleaver must have low loss, preferably less than 3 dB, and it must be approximately characterized by rectangular transfer functions.
A technique that is often used for combining and separating signals of various channels in wavelength-division multiplexing is the wavelength router. A rectangular transfer function can be realized by concatenating two such routers, but such an arrangement is difficult to realize on a single wafer using conventional techniques for two reasons. The first reason is that the loss of a conventional router typically exceeds 3 dB, which would result in two concatenated routers having a total loss of more than 6 dB. Another reason is that the two-router arrangement is difficult to realize on a planar wafer because of the size of the two routers, particularly when the channel spacing is equal to the free-spectral range of the routers, in which case one finds that the layout of the two routers typically overlap.
Thus there is a continuing need for a low loss interleaver or slicer having rectangular wavelength transfer functions.
In one embodiment of my new invention, a low loss wavelength slicer is implemented using a waveguide grating router connected to an input 2xc3x972 coupler. The router is terminated by an output reflective arrangement causing each reflected signal to pass twice through the router. The reflective arrangement consists of two interleaved sets of reflective elements located in the immediate vicinity of the output image curve of the router. Each set is slightly displaced from the output curve, and their displacements are properly chosen so as to effectively produce a phase shift of xcfx80/2 between the signals reflected by the two sets. The reflected signals then pass again through the router and the 2xc3x972 coupler which produces two separate output signals, containing respectively the even and odd channels of the input signal, which are each outputted from separate input ports of the 2xc3x972 coupler. In another embodiment, a wavelength filter is realized by forming on the output curve a set of equally spaced reflectors with spacing equal to an integer fraction of the spacing of the images. A second slicer embodiment is realized by using two routers or gratings that are coupled to elliptical and circular reflectors that separate the even and odd channels.
More generally my invention is directed to an optical imaging apparatus comprising a grating having an input curve and an output image curve produced with specified magnification M, the optical imaging apparatus further comprising
(1) an input waveguide connected to a location P of the input curve for receiving an input signal at a particular input wavelength, the input signal being dispersed by the various orders of the grating into a set of equally spaced images produced on the output image curve with spacing xcexa9, the dispersion by the grating causing the locations of the output images on the output image curve to vary with wavelength and
(2) a periodic arrangement A of reflective elements located along the output image curve with period essentially equal to an integer fraction of the spacing xcexa9 of the output images, where the integer can be equal to unity, each reflective element essentially reversing (reflecting over the same path) the propagation direction of an image intercepted by that element, the periodic arrangement thereby causing the set of equally spaced images to produce reflected signals which pass back through the grating to the input waveguide, the power reflected signals are efficiently transferred back to the input waveguide, thereby causing an output signal traveling in the input waveguide in a direction opposite that of the input signal.
In a second arrangement, a second input waveguide is connected to a location Q of the input curve, so as to realize two separate input ports with locations P and Q separated by a specified lateral displacement d, the periodic arrangement of reflective elements A is interleaved with a second periodic arrangement of reflective elements B, so that each element A is adjacent to an element B, and the spacing of the two elements is approximately equal to the image Md of the input displacement d, the two elements A and B being characterized by reflections of similar magnitudes but different phases that approximately differ by xcfx80/2.
In another embodiment, the displacement d is chosen so that its image Md on the output curve is approximately equal to half the spacing xcexa9 of the output images, and the grating is arranged to have a suitable periodic path length variation in adjacent arms so as to cause xc2x1xcfx80/4 opposite phase shift in adjacent arms, wherein a particular input signal of a particular wavelength is split into two interleaved output image sets displaced by xcexa9/2. The optical apparatus thereby forms a reflective imaging arrangement with two input waveguides characterized by four reflection coefficients, each reflection coefficient produced in a particular input waveguide by a signal applied to one of the two input waveguides, such that the wavelength response produced by each of the four reflection coefficients is characterized by two sets of interleaved wavelength intervals U and V, with one set representing passbands of efficient reflection, and the other set representing stopbands of substantially lower reflection.
In yet another embodiment, instead of including the above path length variation, a 2xc3x972 coupler is added to the optical apparatus thereby forming a reflective arrangement which is characterized by four reflection coefficients, each reflection coefficient produced in a particular input waveguide by a signal applied to one of the two input waveguides. The reflective arrangement, in response to a multiple wavelength input signal applied to the first input port, produces a reflected signal back to the first input port which has a wavelength response characterized by two sets of interleaved wavelengths U and V, with one set U representing passbands of efficient reflection back to the first input port, and the other set V representing stopbands of substantially lower reflection back to the first input port.